Modems are transceiver devices that allow digital data to be transmitted between pieces of digital equipment, such as computers, via the telephone lines.
Over the past few decades, several standards for communication via modems have been developed. Two of the more recent standards that has been promulgated by the ITU (International Telecommunications Union), formerly known as the CCITT, are ITU-T recommendations V.90 and V.92, incorporated herein by reference.
Most households couple to the local central office of the telephone company through a two wire twisted pair connection. Communication over the two wire twisted pair typically is in analog form. Accordingly, the modem converts the digital data to be transmitted via the telephone network into an analog format that can be transmitted via the twisted wire pair, analog, portion of the telephone network. At the central office of the telephone company, the data is converted into digital format at 64 KB per second and the data is transmitted between central offices in digital format. If the second customer at the opposite end of the telephone call also is coupled to the central office via a twisted wire pair, analog, portion of the network, the data is converted back to analog at the central office closest to the second customer and transmitted to the second customer over the twisted wire pair. The second customer's modem receives the data, converts it back to digital and sends it to the computer.
However, in recent times, many customers of telecommunications services, and particularly any large scale customer of telecommunications services, couple to the central offices through a digital connection, such as a T1 or a T3 connection, well known to those of skill in the art. Certainly, the vast majority, if not all, of Internet Service Providers (ISPs) couple to the telephone company central offices directly in digital.
Generally in the telecommunications industry, as well as in this specification, the following terminology is used. Data transmitted from an individual household customer (subscriber) to an ISP is termed upstream communication. Data transmission from an ISP to a subscriber is termed downstream communication. In accordance with the V.90 protocol, the data format is different in the downstream direction than it is in the upstream direction. In the V.90 standard, modem transmission in the upstream direction is an analog signal in accordance with the older V.34 standard and is transmitted at a maximum data rate of 31.2 kilobits per second (Kbps). However, downstream communication is a PCM (pulse code modulated) signal that can be transmitted at a maximum rate of 56 Kbps per second. In the V.92 standard, communication the both directions is PCM at a maximum rate of 56 kbps.
FIG. 1 is a block diagram generally illustrating modem-to-modem communications through a public telephone network. The system will be described in connection with a public telephone network household customer exchanging data with an Internet service provider (ISP). Let us assume the household customer and the ISP are coupled to different central offices of the public telephone network.
The customer at computer 12 inputs and sends data to the ISP at 28. The computer 12 includes a built-in UART and, therefore, sends out a serial digital signal to the modem 14. The modem converts the serial digital signal to comply with the V.90 standard upstream protocol and puts it out on the public telephone network 20.
Within the telephone network, communication between central offices is digital, rather than analog. Accordingly, the analog signal is encoded by a CODEC 22 into a 64 Kbps signal. In particular, the received analog signal is sampled at a rate of 8 KHz and digitized at an 8 bit resolution to produce a 64 kbps digital PCM signal. The 64 kbps standard is known in the United States as the μ-law standard and in Europe as the A-law standard. The information is digitally transmitted between central office 24 and central office 26.
If the other customer (the ISP) had been coupled to the telephone network through a twisted wire pair, the digital signals received at central office 26 from central office 24 would be passed through another CODEC (not shown) to be decoded back to analog form. The decoded analog signals would then be forwarded to the receiving customer.
However, as previously noted, a high volume customer of the public telephone network, such as ISP 28, would normally have a pure digital connection to the central office 26. Accordingly, ISP 28 would not use a CODEC in central office 26, but instead would receive the data directly in digital form over a digital link such as T1 line 30.
In the opposite direction, ISP 28 outputs digital data to central office 26 via T1 line 30. This data is transmitted in digital form to central office 24. CODEC 22 in central office 24 decodes the digital data and transmits it to the customer's modem 14.
FIG. 2 is a more detailed block diagram illustrating the typical connection between a household and an ISP through a public telephone system. At the household end, the modem 214 includes a transmitter 203, a receiver 205, a CODEC 209 and a hybrid circuit 208. Within the modem, there are separate transmit and receive data paths. Accordingly, digital data from transmitter 203 is transmitted over transmit path 204 to CODEC 209. CODEC 209 converts the data from digital to analog for transmission over the twisted wire pair 211. In the receive direction, CODEC 209 converts data received over the twisted wire pair 211 from analog to digital and transmits it over the receiver path 210 to the receiver 205. Since the analog portion 211 of the public telephone network, to which the household customer directly couples, is a two wire, analog system, the modem 214 includes a hybrid circuit 208 to interface between the CODEC 209 and the analog portion of the public telephone network 211. In the transmit direction, hybrid circuit 208 takes the transmit (i.e., upstream) data on transmit path 207 from the CODEC 209 and places it on the two wire portion 211 of the telephone network. In the downstream direction, hybrid circuit 208 selects and isolates the downstream data, on transmit on the two wire portion 211 of the telephone network and forwards it to the CODEC 209 on the receive wire path 213.
There is almost always an impedance mismatch between the customer's telephone equipment and the public telephone network. This impedance mismatch has the unfortunate effect of causing an echo at the hybrid circuit 208. The echo occurs in both directions. For instance, data transmitted from the modem 214 through the hybrid 208 is reflected back on the receive path 210 in the modem as illustrated by arrow 212.
Likewise, downstream data from the ISP via the public telephone network also is reflected at hybrid 208, back to the ISP, as illustrated by arrow 215.
At the central office 229, there is another hybrid circuit 224 and CODEC circuit 226 serving essentially the same functions as the aforementioned hybrid circuit 208 and CODEC 209. Second hybrid circuit 224 is the interface between the analog two wire portion 211 of the public telephone network and the digital, four wire inter-central-office portion 217 of the network. Hybrid circuit 224 also creates echos in both directions. The echo 225 from hybrid circuit 224 passes back through hybrid circuit 208 and reaches the receive data path 210 in modem 214. Likewise, the ISP also receives a second echo 227 off of the hybrid circuit 224. Accordingly, typically, the customers at both ends link, e.g., the ISP and the household customer, are subject to at least two echos.
Typically, because the hybrid circuit 208 in the customer's own equipment as well as the hybrid circuit 224 in the customer's local central office are physically close to the customer, both of the echos 212 and 225 are almost simultaneous with the actual transmission of the data. Accordingly, both of these echos are herein termed “near echos”. Accordingly, the near echos experienced by the customer's modem 214 and computer can often be a problem. Nevertheless, many modems have near echo canceller circuits to correct for corruption of downstream data by the near echo signals.
Both of these hybrid circuits 208 and 224 typically are relatively distant from the ISP. Accordingly, the two echos 227 and 215 received at the ISP commonly are sufficiently delayed from the original transmission of the data to be more problematic, i.e., to corrupt data on the receive path at the ISP (upstream data) that is received simultaneously with the far echo signals.
The signals travel through the digital portion 217 of the network to the central office 231 local to the modem 235 of ISP 233.
In order to minimize the effect of far and near echo, therefore, a digital loss of approximately six decibels (dB) typically is incorporated into hybrid circuits so as to reduce the amplitude of the echo. However, even with the incorporation of the digital loss, far echo can sometimes still create sufficient noise to corrupt valid data.
Thus, in order to further compensate for echo, digital communications equipment (e.g., modems) commonly include a far echo canceller circuit. FIG. 3 is a block diagram of an echo canceller circuit of the prior art. The transmit signal from transmitter 300 on transmit path 301 is fed out to the digital network 302. The transmit signal also is fed into an echo cancellation circuit 303. The echo cancellation circuit includes a bulk delay line buffer 304 and a Finite Impulse Response (FIR) filter 306. FIR 306 receives the transmit signal from transmit wire pair 301 through bulk delay line buffer 304 and generates an echo cancellation signal that can be used to cancel the far echo signal portion that returns from the network. The FIR circuit 306 determines during a training phase at the beginning of each call, the impulse response for the channel, emulates it, and applies it to the data transmitted from transmitter 300 so that the echo cancellation signal 305 emulates the echo signal. The bulk delay line buffer 304 is the circuit that determines and causes the necessary delay in order to cause the output from the FIR circuit 306 to be simultaneous with the receipt of the far echo.
As is well known in the art, each call starts with a training phase before any real data is transmitted. During the training phase, the run trip delay of the far echo as well as the impulse response of the channel for any given telephone call is determined. Accordingly, a processor 312 in the modem determines the round trip delay and the necessary coefficients for the FIR circuit 306 from the handshaking data and sends the data to the bulk delay line buffer 304 and the FIR, respectively. The delay circuit 304 will then delay passing the transmit data from transmit path 301 to the FIR circuit 306 for the appropriate duration, namely, the round trip delay, and the FIR will attenuate and otherwise condition the transmit signal to emulate the echo signal. Subtractor 310 subtracts the output of FIR circuit 306 from the receive data path 308 in order to cancel the far echo component that appears on receive data path 308.
Another noise factor inherent in telephony communications is “robbed bit” noise. In particular, in the digital portion of the network between telephone company central offices, the least significant bit (LSB) of every sixth data sample is utilized for synchronization. In the United States, for instance, there are two types of robbed bit loss, termed type A and Type B. In type A robbed bit systems, for example, the LSB of every sixth data sample (each data sample comprises 8 bits) is forced to digital one regardless of the actual data content. Further, if a connection is routed through a plurality of central offices between the two termination points of the connection, a robbed bit may be inserted for each central office through which a particular call is routed such that there may be several robbed bits every six samples. As will become clear from the discussion below, the present invention is applicable regardless of the particular robbed bit protocol utilized or the number of robbed bits inserted.
In voice communications, for which, of course, the telephone network was originally constructed, the loss of that bit is imperceptible to the listener and, therefore, unimportant. However, in PCM data communications over the telephone network, the robbed bit must be accounted for. Particularly, data cannot be sent in that bit position since it will be corrupted in the digital portion of the network.
Further, the far echo that comes back through the digital network includes robbed bits. Accordingly, the echo cancellation signal generated by echo cancellation circuit 303 will not exactly match the actual echo signal because the actual echo contains robbed bits, whereas the signal that was transmitted on transmit path 301, and, therefore, was used to create the echo cancellation signal did not contain robbed bits.
U.S. patent application Ser. No. 09/392,380, filed Sep. 9, 1999, assigned to the same assignee as the present application and fully incorporated herein by reference, discloses an improved far echo canceller for PCM modems that includes robbed bit compensation.
FIG. 4 is a block diagram of the front end of a V.90/V.92 standard “central” modem 401. As used herein, the term central modem refers to a modem that couples directly to the digital portion of the network without passing through a two wire twisted pair, analog connection. Thus, a central modem such as might be found in the facilities of an ISP or other large-scale telephony customer that can hook directly to the digital portion of the telephone network transmits and receives in PCM format. Thus, for example, referring to FIG. 1, the central modem would be the modem of ISP 28, which transmits and receives in PCM.
The central modem transmits data on transmission wire pair 402 to the digital network 404. The digital network modifies the signal to insert the robbed bit once every six samples. Thus, when the far echo comes back from the hybrid circuit at the far central office on receive wire pair 406 and the hybrid circuit of the customer's modem, the echo typically is different due to the addition of the robbed bit to the original signal.
A robbed bit may be added in the downstream signal as well as in the echo of the upstream signal. In fact, if a call is routed through several central offices between termination points, several robbed bits may be inserted in each direction. The robbed bits inserted in the upstream direction in the actual echoed signal are of less significance because of the digital loss circuitry which attenuates the echo. Specifically, by the time an upstream robbed bit returns in an echo to the transmission source, it has gone through at least one digital loss circuit and is therefore of almost negligible amplitude. The downstream robbed bit does not experience the digital loss. Thus, the robbed bits added in the downstream direction are the ones that are of more concern to the performance of central ones PCM modems.
The front end of the central PCM modem includes a far echo canceller circuit 410. This far echo canceller comprises a robbed bit generator 412, a bulk delay line buffer 414, a FIR 416 and a subtractor 418.
In order to incorporate robbed bit correction into the echo cancellation scheme, the location of the robbed bit must first be determined. The information necessary to determine the position of the robbed bit is obtained from the other modem at the opposite end of the connection during the training phase at the commencement of a communication link. Particularly, the central PCM modem sends a training signal to the customer's modem. In connection with the receipt of the training signal, the customer's modem detects the position of the robbed bits. The customer's modem then sends the information of the position of the robbed bits back to the central PCM modem. That information is used by the robbed bit generator in the echo canceller circuit 412 to modify the signals it receives from the central PCM modem transmitter to add in the effect of the robbed bit. That modified signal is then sent to the bulk delay line buffer 414.
During the training phase, the central PCM modem also determines the time delay of the far echo by measuring the round trip delay during a portion of the start up protocol in which the customer's modem is not transmitting any data. This allows the central PCM modem to receive the far echo signal without any other data being placed on the line. This measurement is well known in the prior art. The bulk delay line buffer 414 then delays the output of the modified signal to the FIR circuit 416 for the determined round trip delay. The FIR circuit 416 calculates and applies the impulse response of the channel to the signal and outputs an echo cancellation signal to subtractor 418 in order to overlap and cancel the far echo received from the digital data network 404 on receive line 406. The output on line 420, termed the residual signal, is then forwarded to the receiver 424 of the central PCM modem. As illustrated by feedback line 426, the FIR circuit includes feedback for continuously updating the coefficients of the FIR circuit.
Once the position of one robbed bit is determined, then the position of all robbed bits is known since they occur at regular intervals. The central PCM modem digital signal processor 428 also must determine what type of robbed bit protocol is being used on the network. This information also is typically determined during training and is well known in the art. Alternately, the PCM modem may simply be pre-set to a particular type of robbed bit compensation since, frequently, it is known in advance what type of public telephone network the modem would be used in connection with and particularly what type of robbed bit protocol is used on that network.
The position of robbed bits in the downstream direction can be determined by the remote modem in the V.92 protocol.
It would be beneficial to be able to determine the position of the robbed bit and account for it in the echo cancellation scheme without the need to rely on the modem at the opposite end of the link.
Accordingly, the present invention relates to an improved method and apparatus for detecting robbed bit position in the far echo path in a digital communications network.